Unsaturated polyester resins are typically used as the resin component in the manufacture of fiber reinforced thermoset plastics. The resins generally consist of unsaturated polyesters dissolved in a polymerizable ethylenically unsaturated monomer such as styrene. Typically, these unsaturated polyesters are formed by reacting an unsaturated diacid or anhydride with a nearly equivalent amount of dihydric alcohol at temperatures above about 200.degree. C. for several hours. Maleic anhydride is the most common acid component utilized. The dihydric alcohols which are commonly used include ethylene glycol, 1,2-propylene glycol, dipropylene glycol, diethylene glycol, and the like. Modifying amounts of other diacids, such as phthalic acid, isophthalic acid, terephthalic acid, or adipic acid are also commonly employed. Phthalic anhydride is also often used. Unsaturation which is provided by maleate or fumarate groups within the backbone of the polyester takes part in the crosslinking, or curing, of unsaturated polyester resins. These unsaturated polyester resins are considered among the least expensive resins suitable for the manufacture of a variety of fiber reinforced products.
Much effort has recently been directed toward the development of alternative resin systems with potentially improved economics. To be a viable alternative, such resin systems should exhibit the highly desirable properties of conventional unsaturated polyester resins and provide fiber reinforced thermoset articles having good mechanical properties. Modification of unsaturated polyesters with olefins such as dicyclopentadiene has been investigated as one method for reducing the cost of unsaturated polyester resins systems.
Investigators have found that incorporation of dicyclopentadiene results in two structurally different types of polyesters. See, for example, D. L. Nelson, Considerations: Dicyclopentadiene in Polyester Resins, 36th Annular Conference, Reinforced Plastics/Composites Institute, The Society of the Plastics Industry, Inc., Feb. 16-20, 1981, Session 7-E, pages 1-7. One polyester type contains ester and/or ether groups resulting from carboxyl or hydroxyl addition to one of the dicyclopentadiene double bonds. Such ester and/or ether addition groups have the formula ##STR1## The other polyester type contains Diels-Alder reaction groups resulting from dimer cracking and subsequent reaction of cyclopentadiene with a dieneophile such as maleic acid. Such Diels-Alder groups have the formula ##STR2## when maleic acid is the dieneophile. Combinations of the above groups may also be incorporated in polyesters.
U.S. Pat. Nos. 4,233,432 and 4,246,367, both assigned to United States Steel Corporation, disclose dicyclopentadiene modified polyester resins and a method for preparation thereof. The resins are prepared by reacting maleic anhydride and water with dicyclopentadiene at a temperature of about 90.degree. C. to 150.degree. C. to give a maleic half ester of dicyclopentyl alcohol, and thereafter reacting a glycol, e.g., propylene glycol, at a temperature of about 210.degree. C. to form the resultant unsaturated polyester.
U.S. Pat. No. 4,224,430, assigned to Hitachi Chemical Company, Ltd., discloses high solids resin compositions containing one or more oligomers prepared by reacting dicyclopentadiene with an alpha, beta-unsaturated hydrocarbon, e.g., maleic acid or maleic anhydride, at a temperature of from 100.degree. C.-140.degree. C. to yield a monobasic acid, which is further reacted with a polyhydric alcohol, e.g., diethylene glycol, at a temperature of from 180.degree.-220.degree. C. Cast articles were prepared from the resin compositions.
U.S. Pat. Nos. 4,029,848 and 4,148,765, both assigned to Dow Chemical Company, disclose resin compositions containing an unsaturated polyester obtained by reacting (1) a glycol, e.g., propylene glycol, (2) an olefinically unsaturated dicarboxylic acid or anhydride, e.g., maleic anhydride, (3) a saturated dicarboxylic acid or anhydride, e.g., phthalic anhydride, and (4) dicyclopentadiene at a temperature of about 140.degree. C. for a period of time and thereafter at a temperature of about 200.degree. C. for an additional period of time. Glass laminates were prepared from certain of the resin compositions by a hand roller technique.
U.S. Pat. Nos. 3,166,434 and 3,340,327 disclose resin compositions containing an unsaturated polyester obtained by reacting (1) an unsaturated dicarboxylic acid containing a major molar proportion of fumaric acid, (2) a glycol containing a major molar proportion of polyoxyalkylene glycol and (3) dicyclopentadiene at a temperature up to about 215.degree. C. in the absence of a catalyst (see Example 1 in both patents). Diels-Alder reaction products accompany this polyesterification reaction. Coatings were prepared from certain of the resin compositions.
Some polyester resins with potentially improved economics have been developed which impart smooth surfaces to molded products prepared therefrom. Surface smoothness is an important characteristic of fiber reinforced composites for many applications such as automotive body panels and appliance housings. A thermoplastic polymer low profile additive is typically used in resin systems in which molded articles having good surface properties are desired.
U.S. Pat. No. 4,233,413, assigned to Hitachi Chemical Company, Ltd., discloses low shrink resin compositions containing an unsaturated polyester obtained by reacting dicyclopentadiene with an alpah, beta-unsaturated dibasic acid, e.g., maleic acid, or reacting dicyclopentadiene with an alpha, beta-unsaturated dibasic acid anhydride, e.g., maleic anhydride and water at 150.degree. C. or lower to yield a partially esterfied dicyclopentadiene carboxylic acid, which is further reacted with at least one polyhydric alcohol such as a glycol. e.g., propylene glycol, at a temperature of from 150.degree. C.-210.degree. C. This patent discloses cast articles prepared from the resin compositions.
U.S. Pat. Nos. 3,883,612, 3,933,757, 3,986,992, 4,035,439 and 29,555, all assigned to SCM Corporation, disclose thickened, low shrink molding compositions containing a dicyclopentadiene-modified polyester polymer obtained by the following steps: (1) reacting a glycol, e.g., propylene glycol, and an unsaturated dibasic acid, e.g., maleic acid, at a temperature of about 150.degree. C. to yield an acid terminated partial copolymer; (2) reacting dicyclopentadiene with the partial copolymer prepared in step (1) at a temperature of about 150.degree. C. to yield a prepolymer; (3) reacting additional glycol with the prepolymer of (2) at a temperature of about 200.degree. C. to yield a dicyclopentadiene polyester polymer; and (4) adding styrene to the dicyclopentadiene polyester polymer. Sheet molding compounds (SMC) and bulk molding compounds (BMC) were prepared from certain of the molding compositions.
In all of the above prior art methods, dicyclopentadiene or its reaction product is present in the reaction mixture with an unsaturated diacid or anhydride and a dihydric alcohol at temperatures of from about 150.degree. C. to about 220.degree. C. At these temperatures, both reacted and unreacted dicyclopentadiene become increasingly susceptible to fragmentary side reactions. For example, the remaining double bond in reacted dicyclopentadiene becomes increasingly susceptible to esterification and etherification reactions. Also, the formation of carbic acid can potentially occur at these high reaction temperatures. Any unreacted dicyclopentadiene can readily undergo dimer cracking at temperatures above about 150.degree. C. and thereby provide for the formation of Diels-Alder groups which can be incorporated into the structure of the polyester. Gelation of dicyclopentadiene modified polyesters has occasionally been observed during polyesterification at temperatures of from 150.degree. C. to 220.degree. C., apparently due to side reactions involving the olefin. See, for example, Comparative Example A hereinbelow. Therefore, it would be highly desirable to prepare dicyclopentadiene modified polyesters by a process in which dicyclopentadiene is selectively reacted with a polyester having terminal fumarate half ester groups at low reaction temperatures. It would also be highly desirable to provide low shrinking curable resin compositions containing such dicyclopentadiene modified polyesters which would impart reduced surface roughness to fiber reinforced composites prepared therefrom.
It has been found as a result of this invention that dicyclopentadiene can be selectively reacted at low reaction temperatures with a polyester having fumarate half ester groups by utilizing a non-oxidizing acid catalyst having a non-nucleophilic anion. It has further been found that resin compositions containing (i) the dicyclopentadiene modified polyesters of this invention, (ii) an ethylenically unsaturated monomer, and (iii) a selected thermoplastic polymer low profile additive impart decreased surface roughness to fiber reinforced composites molded therefrom. These low shrinking resin compositions may be used in a rapid injection molding process, sheet molding compound (SMC) and bulk molding compound (BMC).
Dicyclopentadiene need no longer be exposed to high reaction temperatures utilized in the prior art methods, and therefore dicyclopentadiene is significantly less susceptible to fragmentary side reactions. The dicyclopentadiene modified polyesters of this invention contain predominantly terminal ester groups resulting from selective acid addition to one of the dicyclopentadiene double bonds. The formation of Diels-Alder reaction products is selectively minimized by utilizing dicyclopentadiene at low reaction temperatures.
There appears to be no prior art directed to the addition of dicyclopentadiene at low reaction temperatures by polyesters containing predominantly fumarate half ester terminal groups and, in particular, to low shrinking resin compositions containing such polyesters.
The low shrinking curable polyester resin compositions of this invention exhibit highly desirable properties, e.g., low viscosity, rapid cure rate, excellent moisture resistance, good high temperature properties, and the like, and also provide fiber reinforced composites having good mechanical properties and reduced surface roughness. These low shrinking polyester resin systems are very suitable for use in reaction injection molding of fiber reinforced thermoset articles. The low shrinking curable resin compositions exhibit faster cure rates than the resin systems of U.S. patent application Ser. No. 626,146, now U.S. Pat. No. 4,532,297, filed on an even date herewith.